Hormonal Influences on Brain Development and Behavior

Differences between the sexes have been observed across multiple domains of cognitive, emotional, and behavioral development. Boys, for example, appear on average to be predisposed to more physical activity; less tolerance for frustration; and more aggression, impulsivity, and dys-regulated emotions (Eaton and Enns, 1986; Else-Quest, Hyde, et al., 2006; Zahn-Waxler, Shirtcliff, and Marceau, 2008). Girls on average exhibit more rapid language acquisition, greater empathy and social skills, and more fearfulness and anxiety (Else-Quest, Hyde, et al., 2006; Zahn-Waxler, Shirtcliff, and Marceau, 2008).

Several processes, ranging from differences in environmental exposures to innate differences in the biological processes that underlie either emotion and behavior or responses to the environment, could produce these gender differences (Zahn-Waxler, Shirtcliff, and Marceau, 2008). The differences are thought to have their basis at least in part in differences in brain structure and function, which are determined largely by the effects on brain development of both sex hormones and genes encoded on sex chromosomes (Arnold, 2004; Davies and Wilkinson, 2006; Hines, 2003). Hormone-dependent sexual differentiation of the brain is thought to be driven primarily by differences in androgen levels in fetal and early postnatal life. Production of testicular androgen in the human male fetus begins during the sixth week of gestation, producing higher testosterone levels in males than in females between weeks 8 and 24 of gestation (Knickmeyer and Baron-Cohen, 2006; Warne and Zajac, 1998). Studies in animal models demonstrate that differences between the sexes in the levels of various steroid hormones in the brain during fetal life produce sex-specific differences in neuronal proliferation, cell migration, apoptosis, dendritic branching, and the density of dendritic spines (Cooke, Hegstrom, et al., 1998). These differences between the sexes in fetal brain development in turn produce gender differences in brain form and structure that endure throughout postnatal life (Knickmeyer and Baron-Cohen, 2006; Hines, 2003). Changes in levels of steroidal hormones during puberty are then thought to lead to further modification of brain structure and function across both sexes (Romeo, 2003).

Differences between the sexes in brain structure and function are thought to underlie the well-documented gender differences in the diagnostic and age specificity of MEB disorders. For example, females overall are more likely than males to develop major depression and anxiety disorders (Pigott, 1999; Rutter, Caspi, and Moffitt, 2003; Zahn-Waxler, Shirtcliff, and Marceau, 2008), while males are more likely to develop ADHD, conduct disorder, substance abuse, tic disorders, and learning disorders (Rutter, Caspi, and Moffitt, 2003; Zahn-Waxler, Shirtcliff, and



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